I love my KRISS CRB. They now allow civilians to get the barrel shortened to convert it to a SRB but it costs $400 plus an ATF sticker $200 for the Class III. I decided to skip the ATF and keep the longer barrel for accuracy and distance.
I would think the dirtiness is the ammo you are using. I hate S&W .45 ammo and avoid it. It feels like grit is on the gun after using S&W. I use mainly blazer brass or Remington ammo for plinking. I bought a couple 30 round magazines (1 speed loader) and they work fine. A couple are loaded with 230grain Federal Hydrashok just in case I have a posse coming for me. :-)
I would have loved to get an Eotech ontop but blew all my cash on the KRISS at the time. Over time I added a Leupold scope and a foregrip. My first foregrip was a springloaded bipod but I moved that to another rifle and got a Sig Arms STL-300J Joint Defense Light. It has a very bright light and a laser ontop of the grip. My complaints about this grip is all the on/off buttons are on the right side which is hard to reach with a left hand. It has an activation button on the back where your thumb rests though which is perfect. So with the laser and strobe for short range and the scope for long range, I think I have my targets covered.
Breaking it down is very easy and putting it back together is mostly fool-proof. My biggest complaint is that as a civilian I can't own the SMG fully automatic version. :-)